A CDMA cellular network is a digital spread spectrum communications system. The CDMA network includes several base stations each providing digital service to mobile units located in different geographical regions. Communication between a mobile unit and a base station in a CDMA network occurs on reverse and forward CDMA channels. The reverse channel is a mobile unit-to-base station direction of communication that carries traffic and signaling information. The forward channel is a base station-to-mobile unit direction of communication that carries pilot, sync, and paging signals in addition to traffic signals.
CDMA networks are deployed differently from analog and other digital networks. CDMA networks are typically deployed with the ability to operate at only one frequency because additional frequencies, though possible, require additional hardware. To increase coverage area, conventional CDMA networks use one common frequency transmission and soft handoff mechanisms. This results in approximately a 3-4 dB advantage in the transmission channel budget, and thus, a larger cell site size for deployment purposes.
Handoffs typically occur when mobile or portable units traverse the cell sites of the network. A "soft handoff" is when the mobile unit begins communication with a new base station on the same CDMA frequency assignment before terminating communication with the old base station. A "hard handoff," on the other hand, is characterized by a temporary disconnection of the mobile unit's Traffic Channel. Hard handoffs occur, for example, when the CDMA frequency assignment changes or when the mobile unit is directed from a CDMA Traffic Channel to an analog voice channel.
The capacity of a CDMA network is a complex function of the handoff condition of the network, the propagation environment, and the loading of the surrounding cell sites. In a typical CDMA network, the capacity on the reverse channel is limited by interference from mobile units operating in other cell sites. This cuts the capacity on the reverse channel by about 30 to 40% in a typical propagation environment with a pathloss exponent of 3.5 to 4. The reduction in capacity on the reverse channel is measured in terms of a frequency reuse factor. FIG. 1 is a graph of the frequency reuse factor as a function of the pathloss exponent in a CDMA network deployed at a single frequency.
In some circumstances, intracell interference significantly reduces the capacity of the CDMA network, especially when the propagation coefficient is very low (i.e., pathloss exponents of 1 or 2, which equates to 10 or 20 dB reductions in signal strength per decade change in distance). In these circumstances, a majority of the signal received at the base station is a result of interference from other cell sites. This limits the capacity of the CDMA network.
The maximum reverse channel capacity, or pole capacity, is a function of many CDMA parameters and can be expressed approximately by ##EQU1##
where N represents the number of users, W is the signal bandwidth (e.g., 1.23 MHz), R is the information rate (e.g., 9.6 kbps or 14.4 kbps for Rate Set I or Rate Set II), E.sub.b /N.sub.o is the received signal relative to the noise level (typically 7 dB), d is the voice activity factor, F is the frequency reuse factor (i.e., the ratio of in-band to out-of-band interference), and G is the sectorization gain (e.g., 2.55 for a 120 degree antenna).
A typical set of parameters, where d=40%, R=9.6 kbps, and F=0.6, results in 38 channels per frequency channel (1.23 MHz). CDMA networks typically operate at 50% reverse channel capacity, which means that the network realizes approximately 19 traffic channels per carrier.
The frequency reuse factor (F) is defined as ##EQU2##
Intracell Interference represents interference caused by mobile units operating within the cell site, and Intercell Interference represents interference caused by mobile units operating within other cell sites. Intercell Interference detracts from the usable capacity of the network, and must be reduced to increase the frequency reuse factor to its theoretical maximum of unity.
Today, CDMA network operators are granted a greater frequency bandwidth than actually used to deploy the CDMA network. However, these operators continue to deploy a single frequency and use the benefits of soft handoffs to improve call quality. Soft handoffs are typically employed for two reasons: (1) to increase coverage, and (2) to improve the handoff process. Where the cell sites are capacity limited, such as in high traffic areas, however, there is a reduced need for the advantages of soft handoff. As a result, the CDMA networks continue to have limited capacity.
Therefore, a need exists to increase the capacity of the CDMA networks deployed at a single frequency.